They aren't Japanese companies like SoftBank, so they don't need to be as concerned specifically with the yen. Though you can be sure that Apple or any large company with foreign cash flow actively manages currency risk.
You're right that companies diversify into other asset classes, but cash is still a major asset class and that risk needs to be managed.
In contrast, Apple and Qualcomm sell product all over the globe and are currency diversified by default. Softbank revenue on the other hand is overwhelmingly in yen, hence the vast diversification into foreign companies. Since they don't export much they also miss out on higher margins internationally if the yen were to fall.
Also, the BOJ's negative rate policy allows SoftBank to raise cash cheaply with uncommonly low yield A-rated bonds, effectively subsidizing their foreign buying spree.
This. BOJ is propping up Japanese companies, with negative interest at times, in the hopes they can finally get out of their 20 year deflationary economy and the coming demographics issues. Not to say Japan is suffering horribly, but the odds of Japan growing organically through innovation and execution as an economy are practically nil, so a strategy like Softbank's is essential to their economy long-term, and they have the strategic foresight and positioning (they are one of the largest telcos remember) to do this.
Softbank also has the balls to make investments like this. The ARM investment was basically a bet on everything hardware-based for the next 20 or 30 years, so clearly they're good at picking the strategic foundations.
Traditionally, Japanese companies and culture is extremely conservative and consensus driven, and one of Softbank's advantages is that they make these calls.
I think we mostly agree - what you describe is what I was meant when I said actively managing currency risk. Every sale in Japan for Apple is a pile of Yen (real or virtual), that needs to be dealt with, and just holding it brings a lot of risk. You move it, you buy options to hedge if you hold it, you buy stuff with it.
